Nina's doing that too Anna, not feeding properly when breastfeeding! I've recently started Nina on a sippy cup with cooled boiled water and I have the teat cold for her and she will sit there for ages chewing on the teat, must be helping her gums. A girlfriend also once told me this to try:
put some water in a bottle with the teat on, turn it upside down and freeze it, then once frozen have bub suck on the teat. That works with Nina, she loves anything cold and hard.